Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, generator, gearbox, nacelle, and one or more rotor blades. The rotor blades capture kinetic energy from wind using known airfoil principles and transmit the kinetic energy through rotational energy to turn a shaft coupling the rotor blades to a gearbox, or if a gearbox is not used, directly to the generator. The generator then converts the mechanical energy to electrical energy that may be deployed to a utility grid.
To ensure that wind power remains a viable energy source, efforts have been made to increase energy outputs by modifying the size and capacity of wind turbines. One such modification has been to increase the length of the rotor blades. However, as is generally understood, the loading on a rotor blade is a function of blade length, along with wind speed and turbine operating states. Thus, longer rotor blades may be subject to increased loading, particularly when a wind turbine is operating in high-speed wind conditions.
During the operation of a wind turbine, the loads acting on a rotor blade are transmitted through the blade and into the blade root. Thereafter, the loads are transmitted through a pitch bearing disposed at the interface between the rotor blade and the wind turbine hub. Typically, the hub has a much higher stiffness than the rotor blades. Thus, due to the stiffness differential between the hub and the rotor blades, the pitch bearings are often subjected to extreme, varying and/or opposing loads. For example, the inner race of each pitch bearing (i.e., the portion typically coupled to the rotor blades) may be subjected to varying, localized loads resulting from flapwise or edgewise bending of the rotor blades, whereas the outer race of each pitch bearing (i.e., the portion typically coupled to the hub) may be subjected to lower and/or differing loads. This variation in loading across the inner and outer races can result in substantial damage to the pitch bearings caused by high bearing contact stresses, high blade root resultant moments, and hard pressure spots.
Various systems and methods have been employed to control such varying loads in an effort to protect the pitch bearing. For example, one method involves loosening the nuts on the bolts in line with the hard pressure spots such that gaps are created when the pitch bearing is overloaded. Such a method, however, tends to overload adjacent bolts and is therefore not very effective.
Accordingly, an improved system and method for mitigating loads in a pitch bearing, such as ball and raceway bearing contact stresses, would be desired in the art. For example, a rotor blade assembly having a shim plate configured to mitigate bearing contact stresses would be advantageous.